1. Field of the Invention
The present invention relates to a device for controlling steering characteristics (i.e. handling characteristics) of a vehicle such as an automobile, and more specifically, to such a device that controls steering assist torque when a vehicle falls into understeer condition.
2. Description of Prior Art
In a modern power steering system equipped on a vehicle, torque applied to steered wheels for assisting a steering action of a driver (steering assist torque) is controlled based upon parameters of running conditions of the vehicle, such as a vehicle speed. So far, in addition to the purpose of the reduction of torque required for rotating a steering wheel (steering torque), steering control devices are variously designed so as to provide a driver with comfortableness and safety in driving a vehicle.
In some of power steering systems, steering control devices modify steering torque, felt by a driver, for avoiding deterioration of vehicle-running behavior, such as understeer (drift-out) condition, by regulating steering assist torque using information of vehicle running behavior obtained with sensors. Japanese Patent Laid-Open Publication No. 11-20728, for instance, discloses a steering control device of an electric power steering (EPS) system, which is designed to decrease steering assist torque in direction of turning a vehicle in response to the turning vehicle falling into understeer condition. Through making it hard to rotate a steering wheel in the turning direction, this control device warns a driver against rotating a steering wheel further in the turning direction, preventing the vehicle behavior from getting worse.
According to a conventional control strategy of a steering control device as described above, it is difficult for the driver to feel an actual condition of the vehicle and its variation because steering torque is modified to be simply increased in response to a vehicle falling into understeer condition. Without such a modification, steering torque no longer increases when the vehicle falls into understeer condition (steering torque is the reaction from a road surface gripped with a tire on a steered wheel so that steering torque is saturated in understeer condition). Thus, a skilled driver, who knows such characteristics of steering torque, could notice a limit of the turning of a vehicle, where tires on steered wheels can increase no more centripetal force (understeer condition), from slight variation or reduction of steering torque during the rotating of the steering wheel. However, according to a simple increase of steering torque through the steering torque modification in the conventional control strategy, it would be difficult for a driver to feel the limit of the turning of a vehicle because no reduction of steering torque occurs even when a vehicle falls into understeer condition.
Further, in prior art as described above, there is no proposal of the way of determining an increment (decrement) of steering torque (steering assist torque) during its modification for understeer condition. Steering operation of a driver varies dependent upon vehicle running conditions, a road condition, a driver's skill, etc. In order to render the modification of steering torque compatible with variable driver's steering operation, the increment of steering torque in its modification should be determined taking into account steering operational condition together with vehicle running conditions.
Accordingly, a conventional steering control device may be improved more appropriately in conjunction with the modification of steering torque for avoiding and/or escaping from understeer condition.